The present invention relates generally to field-ionizing and electron-emitting structures utilizing field emission cathodes, as described in detail in U.S. Pat. Nos. 3,665,241; 3,755,704; 3,789,471; and 3,812,559, hereinafter referred to as the "Spindt" patents, which patents are incorporated herein by reference. The present invention relates particularly to a specifically improved method for making structures in a way which improves their operation.
Referring immediately to FIG. 1, an electric field producing structure, as disclosed in the Spindt patents and the prior art generally, is diagrammatically illustrated. This structure, which is generally indicated by the reference numeral 10, includes upper and lower generally planar electrodes 12 and 14 spaced apart in parallel confronting relationship to one another with a dielectric layer 16 therebetween so as to insure that the two electrodes are electrically insulated from one another. The lower electrode 14 may be self-supporting, or it may be mounted on a structural support base 18.
The overall electric field producing structure 10 includes a series of closely spaced apertures extending through upper electrode 12 and dielectric layer 16 so as to expose an upper surface segment of lower electrode 14. One such aperture is illustrated in FIG. 1 at 20. Note that the uppermost rim 22 of this aperture is actually the upper electrode's circumferential edge defining the top periphery of aperture 20. Note further that rim 22 is exposed to the surface segment of lower electrode 14 defining the bottom of the aperture. That surface segment is generally indicated at 24.
Still referring to FIG. 1, each aperture 20 contains a single electrically conductive protuberance 26 stemming up from surface segment 24 of electrode 14 such that its uppermost pointed end 28 is substantially coplanar with but spaced from rim 22. In an actual embodiment of the prior art, the apertures 20 are circular in cross-sectional configuration and each of the protuberances 26 is either cone shaped as shown, or includes a cylindrical base or pedestal with a cone shaped upper section. As will be seen hereinafter, the present invention is equally applicable to either shape and, in fact, other configurations. In this regard, it should be noted that the thicknesses of at least upper electrode 12 and dielectric layer 16 and the sizes of aperture 20 and protuberance 26 have been exaggerated for purposes of illustration. In an actual embodiment of the prior art, electrode 12 is approximately 1000-4000 angstroms thick, dielectric layer 16 is approximately 1 micron thick and, the diameter and depth of aperture 20 is approximately 1 .mu.m. Thus, the base of the protuberance 26 is very slightly less than 1 .mu.m while its height corresponds to the depth of aperture 20.
Having described dielectric field producing structure 10 generally, it is to be understood that a more detailed description may be found in the prior art generally and in the above recited Spindt patents specifications. As discussed in these patents, the structure 10 can be used as either an electron emitting source or as an electric field-ion producing source. In the former case, the lower electrode 14 and protuberance 26 are connected to a negative potential relative to the upper electrode 12 which is typically connected to a positive potential so that each of the protuberances 26 functions as an electron producing cathode, as illustrated in FIG. 1. The reverse is true when the structure functions as a field-ion producing force. That is, the lower electrode 14 and each of the protuberances 26 would be connected to a positive potential and the upper electrode 12 would be connected to a negative potential, in which case overall structure could function as a field-ionizing device. Both such arrangements are discussed in the prior art and reference is made thereto.
In order to further appreciate the present invention, it is important to briefly explain how protuberances 26 are formed within apertures 20 in accordance with the prior art generally and certain ones of the previously recited Spindt patents in particular. As described there, a physical evaporative deposition process is utilized while the overall structure is rotating about an axis normal to the electrodes 12 and 14, as indicated by arrow 30 in FIG. 2. A suitable masking material, for example, aluminum oxide is deposited at a shallow grazing angle, as indicated by arrow 32, onto the upper surface of upper electrode 12 and around each rim 22 so as to form a release layer 34. This release layer also defines the base diameter of its associated protuberance 26, as will be seen. After formation of release layer 34, and while the overall structure is still rotating, deposition of the masking material at the same shallow grazing angle continues simultaneously with the deposition of an electrically conductive material, for example molybdenum, into a apertures 20, above the masking layer and in a direction normal to the electrodes, that is, along the axis of the apertures, as indicated by the arrows 36. As this step continues, the upper opening into each aperture continues to close, resulting in the formation of a single protuberance 26 in each aperture, which protuberance is cone shaped, as stated previously. It is possible to form the base of the protuberance into the shape of a cylinder by initially depositing the electrically connected material into each aperture without first depositing masking material. In either case, after the protuberances are shaped, all of the masking and electrically conductive material deposited on the upper electrode may be readily removed by means of chemically etching away the release layer in order to expose the apertures and protuberances.
Having described the prior art electric field producing structure 10 and its method of manufacture, certain points should be noted. First, it is important to note that each of the protuberances 26 operates most efficiently as an electron emitting cathode when its tip is coplanar with rim 22, as shown in FIG. 1, and when its tip is as close as 25 possible to the rim 22 without actually short-circuiting. By more efficient is meant that the cathode can deliver a greater current for a given voltage under these conditions than would be the case if the tip of the protuberance were below and/or further from the rim. At the same time, when operating in a continuously pulsed mode, this efficiency of operation increases with a decrease in capacitance between the upper and lower electrodes 12 and 14, respectively, and it decreases in efficiency with an increase in capacitance. The capacitance referred to is the capacitance C diagrammatically depicted in FIG. 1 by dotted lines.
With the discussion immediately above in mind, it should be further noted that by the nature of the protuberance 26 formation process it is difficult or impossible to form protuberances 26 with a height H much greater than the aperture diameter D as shown in FIG. 1. With this dimensional relationship, each protuberance 26 is automatically formed such that its height is approximately equal to its base and, more important, it is approximately equal to the height of its aperture, thereby placing its tip in line with the adjacent rim 22 of upper electrode 12. However, because the diameter D of the aperture is equal to its height H, the distance R between tip 28 and rim 22 is relatively large, specifically one-half the height H.
As may be recalled from the discussions above, it is desirable to make the distance between the tip 28 of the protuberance 26 and rim 22, that is, the distance R as small as possible. One way to do that without changing the formation process described above in conjunction with FIG. 2 above is to reduce the diameter D and height H of each aperture while retaining a ratio equal to one between the two in order to ensure that the tips of the protuberances are formed coplanar with rims 22. While this clearly reduces the dimension R since the radius of the aperture itself is reduced, the corresponding reduction in height H between the two electrodes 12 and 14 causes the capacitance C to increase which, as stated above, is undesirable. Thus, reducing the size of the hole in this way is not a satisfactory method of reducing the distance between the tip of each protuberance and its associated rim. In view of the foregoing, it is a primary object of the present invention to decrease the distance between the tip 28 of each protuberance 26 described above and its adjacent upper electrode rim 22 without causing an increase in the capacitance between the upper and lower electrodes.
A more particular object of the present invention is to provide an uncomplicated and yet reliable process for meeting the primary object just recited without departing drastically from the prior art formation process described above in conjunction with FIG. 2.
Another particular object of the present invention is to provide protuberances which are specifically configured to concentrate more of the electric field between the upper and lower electrodes at the tips of the protuberances than heretofore achievable using the prior art formation process described above.
As will be disclosed in more detail hereinafter, the electric field producing structure disclosed herein utilizes upper and lower electrodes and an intermediate dielectric layer corresponding to electrodes 12 and 14 and layer 16 described previously and it includes similar apertures. However, the apertures formed in the structure of the present invention are smaller in diameter D but retain the same depth or height H. This decreases the distance R between the tip 28 of each protuberance and its associated rim 22 without having to decrease the height H and thereby cause the capacitance to increase. This of course assumes that the protuberances 26 can be formed within the narrower apertures so that their tips line up with rims 22. As will be seen hereinafter, this is accomplished by means of a two stage physical evaporative deposition process as compared to the single stage process described above.